Wakizashi shaped Hira Zukuri 平造脇差 (without shinogi), attributed to Kai Mihara School 貝三原, in Mihara in the former province of Bingo 備後, now Onomichi in Hiroshima Prefecture. This unsigned blade is possibly the work of blacksmith Masayoshi 正賀, active around 1495 to 1530, with the known signature ‘Bingo No Kuni Mihara Kai Masayoshi’ 備後國三原住貝正賀.

The Mihara school would have been formalized at the end of the Kamakura period (mid 14^th century) by Masaie 正家, we speak of Ko-Mihara, the branch of departure of this school. It seems however that there was a blacksmith named Masaia who lived in Mihara around 740 AD, there is no existing production but it is still possible.
The Kai Mihara lineage refers to these Mihara blacksmiths during the Muromachi era. We then speak of Sue Mihara, late school Koto. The blacksmiths of Mihara worked in collaboration with the blacksmiths of Bizen, particularly Osafune. But their origins are mainly influenced by the Tegai lineage of the Yamato tradition. In fact, the quenching lines are mainly Suguha. Their blades were known for their sobriety, but especially for the quality of their cutting edge and durability. Kai Mihara School blades are quite rare because they were widely used during the battles of the Muromachi and Sengoku Jidai era.

The other explanation of the scarcity of the blades and the almost absence of production during the Shinto era comes from the fact that the lords (Daimyo) of this part of Japan had been beaten in Sekigahara by Tokugawa and who redistributed the fiefs, depriving them of their wealth at the same time.